Polyethylene molding compound suitable as a pipe material with excellent processing properties

ABSTRACT

The invention relates to a polymeric molding compound made from a first ethylene polymer (A) and from a second ethylene polymer (B) which is particularly suitable for producing thick-walled large-caliber pipes, wherein the molding compound comprises an amount in the range from 55 to 75% by weight of the first ethylene polymer (A) and an amount in the range from 25 to 45% by weight of the second ethylene polymer (B), based in each case on the total weight of the molding compound, where the first ethylene polymer (A) is a copolymer of ethylene with a 1-olefin having a total number of carbon atoms in the range from 4 to 10 as comonomer, and with a proportion of from 0.2 to 5% by weight of comonomer, based on the weight of the first ethylene polymer (A), and with a wide bimodal molar mass distribution, and where the second ethylene polymer (B) is a copolymer made from ethylene units and from a 1-olefin having a number of carbon atoms in the range from 4 to 10, which has a bimodal molar mass distribution differing from that of the first ethylene polymer (A). The invention further relates to a high-strength pipe made from this molding compound, and to its use for the transport of gas or water.

The present invention relates to a polymeric molding compound made froma first ethylene polymer (A) and from a second ethylene polymer (B). Theprocessing properties of the molding compound make it particularlysuitable for producing thick-walled, large-caliber pipes.

Polyethylene is widely used for producing pipes, e.g. for gas transportor water transport systems, because pipes of this type require amaterial with high mechanical strength, high corrosion resistance, andgood long-term resistance. Numerous publications describe materials witha very wide variety of properties, and processes for their preparation.

EP-A-603,935 has previously described a molded compound based onpolyethylene and having a bimodal molar mass distribution, and intended,inter alia, to be suitable for producing pipes. However, pipes producedfrom the molding compounds of that reference are highly unsatisfactoryin relation to their long-term resistance to internal pressure, theirstress-cracking resistance, their low-temperature notch impact strength,and their resistance to rapid crack propagation.

In order to obtain pipes with balanced mechanical properties andtherefore with an ideal combination of properties, it is necessary touse a polymer with still broader molar mass distribution. A polymer ofthis type has been described in U.S. Pat. No. 5,338,589, and is preparedusing a high-activity Ziegler catalyst which is known from WO 91/18934,the magnesium alkoxide used there being a gel-type suspension.

A disadvantage with the processing of the known molding compounds isthat their melt strength is too low. This becomes noticeableparticularly during processing to give pipes. A specific risk apparentduring that process is that the pipe breaks open while molten or duringconsolidation of the pipe, e.g. in a vacuum calibrator unit. Inaddition, the low melt strength frequently leads to continuousinstability of the extrusion process. Furthermore, when the knownmolding compounds are processed a problem of sagging arises duringextrusion of thick-walled pipes. The problem is that specified thicknesstolerances cannot be complied with during industrial manufacture sincethe total time required for consolidation of the pipes fromthermo-plastic is up to a number of hours and the dead weight of themelt therefore causes uneven wall thickness measured around the entirecircumference of the pipes.

It was therefore an object of the invention to provide a polyethylenemolding compound which has sufficiently high melt strength to permit itsuse for producing large-caliber, thick-walled pipes with no risk ofbreak-open of the pipes during production or of the problem of sagging,but at the same time with mechanical properties and product homogeneitywhich are sufficient to comply with the quality criteria for the pipes,such as long-term resistance to internal pressure, high stress-crackingresistance, low-temperature notch impact strength, and high resistanceto rapid crack propagation.

This object is achieved by way of a molding compound of the type statedat the outset, the characterizing features of which are that the moldingcompound comprises an amount in the range from 55 to 75% by weight ofthe first ethylene polymer (A) and an amount in the range from 25 to 45%by weight of the second ethylene polymer (B), based in each case on thetotal weight of the molding compound, where the first ethylene polymer(A) is a copolymer of ethylene with a 1-olefin having a total number ofcarbon atoms in the range from 4 to 10 as comonomer, and with aproportion of from 0.2 to 5% by weight of comonomer, based on the weightof the first ethylene polymer (A), with a wide bimodal molar massdistribution, and where the second ethylene polymer (B) is a copolymermade from ethylene and from a 1-olefin having a number of carbon atomsin the range from 4 to 10, which has a bimodal molar mass distributiondiffering from that of the first ethylene polymer (A).

The molding compound of the invention is prepared by mixing thecomponents of the mixture, prepared separately from one another, thefirst ethylene polymer (A) and the second ethylene polymer (B), in anextruder in the form of an extruder blend.

The molding compound of the invention, which can be used to manufacturea pipe in compliance with the demanding quality criteria on which theobject of the invention is based, preferably comprises a first ethylenepolymer (A) with a density (measured at a temperature of 23° C.) in therange from 0.94 to 0.96 g/cm³ and comprises a broad bimodal molar massdistribution, where the ratio, within the ethylene polymer (A), betweenthe weight of the low-molecular-weight fraction and the weight of thehigher-molecular-weight fraction is in the range from 0.5 to 2.0,preferably from 0.8 to 1.8. According to the invention, the firstethylene polymer (A) contains small proportions of other comonomerunits, such as 1-butene, 1-pentene, 1-hexene, or 4-methyl-1-pentene.

The bimodality of the first ethylene polymer (A) may be described as ameasure of the position of the centers of gravity of two individualmolar mass distributions, with the aid of the viscosity numbers VN toISO/R 1191 of the polymers formed in two separate polymerization stages.VN₁ of the low-molecular-weight polyethylene formed in the firstpolymerization stage here is from 40 to 80 cm³/g, whereas VN_(total) ofthe final product is in the range from 350 to 450 cm³/g. VN₂ of thehigher-molecular-weight polyethylene formed in the second polymerizationstage can be calculated from the following mathematical formula:${VN}_{2} = \frac{{VN}_{total} - {w_{1} \cdot {VN}_{1}}}{1 - w_{1}}$where w₁ is the proportion by weight of the low-molecular-weightpolyethylene formed in the first stage, measured in % by weight, basedon the total weight of the polyethylene formed in both stages and havingbimodal molar mass distribution. The value calculated for VN₂ isnormally in the range from 500 to 880 cm³/g.

The first ethylene polymer (A) is obtained by polymerizing the monomersin suspension, in solution, or in the gas phase, at temperatures in therange from 20 to 120° C., at a pressure in the range from 2 to 60 bar,and in the presence of a Ziegler catalyst composed of a transition metalcompound and of an organoaluminum compound. The polymerization iscarried out in two stages, hydrogen being used in each stage to regulatethe molar mass of the polymer produced.

According to the invention, therefore, a first ethylene polymer (A) isprepared and contains an amount in the range from 35 to 65% by weight oflow-molecular-weight homopolymer as component (A¹), and contains anamount in the range from 65 to 35% by weight of high-molecular-weightcopolymer as component (A²), based on the total weight of the firstethylene polymer (A).

The low-molecular-weight homopolymer of component (A¹) here has aviscosity number VN^(A1) in the range from 40 to 90 cm³/g, and has anMFR^(A1) _(190/2.16) in the range from 40 to 2000 dg/min. According tothe invention, the density d^(A1) of the low-molecular-weighthomopolymer of component (A¹) is in the range up to a maximum of 0.965g/cm³.

In contrast, the high-molecular-weight copolymer of component (A²) has aviscosity number VN^(A2) in the range from 500 to 1000 cm³/g and adensity d^(A2) in the range from 0.922 to 0.944 g/cm³.

A very useful tool for determining comonomer distribution insemicrystalline polyethylene is preparative TREF (Temperature-RisingElution Fractionation). This is described in Polym. Prep. A, Chem.Soc.—Polym. Chem. Div., 18, 182 (1977) by L. Wild and T. Ryle under thetitle: “Crystallization distribution in Polymers: A new analyticaltechnique”. This fractionating method is based on the different abilityof the individual components of a polymer to crystallize inpolyethylene, and therefore permits the semicrystalline polymer to beseparated into various fractions which are simply a function of thethickness of the crystallite lamellae.

FIG. 1 shows the result of a gel-permeation chromatography study of aTREF fraction at 78° C. of a copolymer typically used as first ethylenepolymer (A) for the molding compound of the invention.

The peak indicated by reference numeral 1 covers thelow-molecular-weight, but highly crystalline, PE fraction, soluble at78° C., while the peak with reference numeral 2 represents thehigh-molecular-weight fraction with high comonomer content, thisfraction being responsible for the large number of “tie molecules”between the crystallite lamellae and for the quality of the moldingcompound of the invention, expressed in terms of its extremely highstress-cracking resistance. The high-molecular-weight copolymer ofcomponent (A²) in the fraction at a temperature of 78° C. frompreparative TREF therefore has an average molar mass, expressed in termsof the weight average M_(w), of ≧180000 g/mol.

The second ethylene polymer (B) present in the molding compound of theinvention is a copolymer of ethylene which likewise has a bimodal molarmass distribution and has an MFR^(B) _(190/5) in the range from 0.09 to0.19 dg/min, a density d^(B) in the range from 0.94 to 0.95 g/cm³, and aviscosity number VN^(B) in the range from 460 to 520 cm³/g.

According to the invention, therefore, a second ethylene polymer (B) isprepared in the form of a reactor blend in the presence of a Zieglercatalyst, and comprises an amount in the range from 15 to 40% by weightof ultrahigh-molecular-weight ethylene homo-polymer as component (B¹)and comprises an amount in the range from 60 to 85% by weight oflow-molecular-weight copolymer with 1-butene, 1-hexene, or 1-octene ascomonomer in an amount of from 1 to 15% by weight, as component (B²),based on the total weight of the second ethylene polymer (B). Theultrahigh-molecular-weight ethylene homopolymer of component (B¹) herehas a viscosity number, VN^(B1), in the range from 1000 to 2000 cm³/g,and the low-molecular-weight copolymer of component (B²) has a viscositynumber, VN^(B2), in the range from 80 to 150 cm³/g.

The molding compound of the invention for the pipe to be produced mayalso comprise other additives besides the first ethylene polymer (A) andthe second ethylene polymer (B). Examples of these additives are heatstabilizers, antioxidants, UV absorbers, light stabilizers, metaldeactivators, compounds which decompose peroxides, or basiccostabilizers, in amounts of from 0 to 10% by weight, preferably from 0to 5% by weight, and also fillers, reinforcing agents, plasticizers,lubricants, emulsifiers, pigments, optical brighteners, flameretardants, antistats, blowing agents, or combinations of these, intotal amounts of from 0 to 50% by weight, based on the total weight ofthe molding compound.

The manner of producing the pipe from the molding compound of theinvention is that the molding compound is first plastified in anextruder at temperatures in the range from 200 to 250° C. and is thenextruded through an annular die and cooled. Pipes made from the moldingcompound of the invention are generally suit-able for all pressureclasses to DIN 8074.

For processing to give pipes, use may be made either of conventionalsingle-screw extruders with smooth feed zone or of high-performanceextruders which have a finely grooved barrel and have a feed withconveying action. The screws are typically designed as decompressionscrews with lengths from 25 to 30 D (D=Ø). The decompression screws havea metering zone in which temperature differences within the melt areevened out, and in which the intention is to dissipate the relaxationstresses produced by shear.

The melt coming from the extruder is first distributed by way ofconically arranged holes around an annular cross section, and then fedby way of a spiral mandrel distributor or screen pack to the mandrel/diering combination. When required, there may also be restrictor rings orother design elements installed to render the melt stream uniform priorto die discharge.

Vacuum calibration is advantageously used for calibration and cooling togive large pipe diameters. The actual shaping process takes place usingslotted calibrator sleeves, manufactured from non-ferrous metal toimprove heat dissipation. A film of water introduced within the inletserves here for rapid cooling of the surface of the pipe to below thecrystallite melting point, and also serves as a lubricating film forreducing frictional forces. The total length L of the cooling section isjudged on the basis of the assumption that the intention is that a meltwhose temperature is 220° C. is to be cooled with the aid of water whosetemperature is from 15 to 20° C. sufficiently for the temperature of theinner surface of the pipe to be not more than 85° C.

Stress-cracking resistance is a feature known previously from EP-A 436520. The process of slow crack propagation can be substantiallyinfluenced via molecular structural parameters, such as molar massdistribution and comonomer distribution. The number of what are calledtie molecules or link molecules is first determined by the chain lengthof the polymer. The morphology of semicrystalline polymers is alsoadjusted by incorporating comonomers, since the thickness of crystallitelamellae can be influenced by introducing short-chain branching. Thismeans that the number of what are known as tie molecules or linkmolecules in copolymers is higher than in homopolymers having comparablechain lengths.

Stress-cracking resistance FNCT of the molding compound of the inventionis determined by an internal test method. This laboratory method hasbeen described by M. Fleiβner in Kunststoffe 77 (1987), pp. 45 et seq.This publication shows that there is a relationship between thedetermination of slow crack propagation in the long-term test on testspecimens with a peripheral notch and the brittle variant of thelong-term hydrostatic strength test to ISO 1167. The notch (1.6 mm,razor blade) shortens crack-initiation time and thus time-to-failure in2% strength aqueous Arkopal N 100 detergent solution acting asstress-crack-promoting medium at a temperature of 95° C. and withtensile stress of 4.0 MPa. The specimens are produced by sawing threetest specimens of dimensions 10×10×90 mm from a pressed plaque ofthickness 10 mm. A razor blade in a notching apparatus (see FIG. 5 inthe Fleiβner publication) specifically made for the purpose is used togive the center of the test specimens a peripheral notch of depth 1.6mm.

Fracture toughness aFM of the molding compound of the invention islikewise determined by an internal test method on test specimens ofdimensions 10×10×80 mm, sawn out from a pressed plaque of thickness 10mm. The razor blade of the abovementioned notching apparatus is used togive six of these test specimens a central notch of depth 1.6 mm. Themethod of carrying out the tests substantially corresponds to the ISO179 Charpy test procedure with modified test specimens and modifiedimpact geometry (distance between supports). All of the test specimensare conditioned to the test temperature of 0° C. for from 2 to 3 h. Atest specimen is then moved without delay onto the support of a pendulumimpact tester to ISO 179. The distance between the supports is 60 mm.The 2 J hammer is released and falls, with the angle of fall adjusted to160° C., the pendulum length to 225 mm, and the impact velocity to 2.93m/sec. To evaluate the test, the quotient in mJ/mm² is calculated fromthe impact energy consumed and the initial cross-sectional area at thenotch a_(FM). The only values here which can be used as the basis for anoverall average are those for complete fracture and hinge fracture (seeISO 179).

Shear viscosity is a very particularly important feature of the polymermelt and represents the flow properties of the polymer extruded inmolten form to give a pipe, these properties being very decisiveaccording to the invention. It is measured to ISO 6721-10, part 10, inoscillating shear flow in a cone-plate rheometer (RDS test) initially atangular frequency of 0.001 rad/s and melt temperature 190° C., and thenat angular frequency 100 rad/s at the same temperature. The two valuesmeasured are then placed in relationship to one another, giving theviscosity ratio η(0.001)/η(100), which according to the invention is tobe greater than or equal to 100.

The examples below are intended for further clarification of thedescription of the invention and its advantages for the skilled worker,in comparison with the prior art.

EXAMPLES 1 TO 9

A first bimodal ethylene polymer (A) was prepared to the specificationof WO 91/18934 using a Ziegler catalyst from example 2, which hadcatalyst component a with operating number 2.2, maintaining theoperating conditions stated below in table 1.

TABLE 1 Reactor I Reactor II Capacity: 120 l Capacity: 120 l Temperature83° C. 83° C. Catalyst feed 0.8 mmol/h — Cocatalyst feed 15 mmol/h 30mmol/h Dispersing agent 25 l/h 50 l/h (diesel oil; 130-170° C.) Ethylene9 kg/h 10 kg/h Hydrogen in gas space 74% by volume 1% by volume 1-Butene0 250 ml/h Total pressure 8.5 bar 2.7 bar

The resultant ethylene polymer (A) had a melt flow index MFI^(A)_(5/190° C.) of 0.49 dg/min and a density d^(A) of 0.948 g/cm³, and hada comonomer proportion of 1.5% by weight, based on the total weight ofthe higher-molecular-weight component.

A second bimodal ethylene polymer (B) was then prepared to thespecification of EP-B-0 003 129. For this, 6.7 kg of ethylene/h and 0.24kg of 1-butene/h were introduced into diesel oil with boiling point inthe range from 130 to 170° C. in a stirred tank over a period of 6 h ata constant temperature of 85° C., in the presence of the Zieglercatalyst described in example 1 of the EP-B. After a reaction time of 3h and 20 min, hydrogen was also introduced under pressure and itsaddition was continued so as to maintain a constant hydrogenconcentration in the region of 60-65% by volume within the gas space ofthe stirred tank during the remaining reaction time of 2 h and 40 min.

The resultant ethylene polymer (B) had a melt flow index MFI^(B)_(5/190° C.) of 0.16 dg/min and a density d^(B) of 0.940 g/cm³.

The first bimodal ethylene polymer (A) was then mixed with the secondbimodal ethylene polymer (B) in an extruder.

The mixing ratios are given in the table given below for examples 1 to9, as are the attendant physical properties of the molding compoundresulting from the mixture:

TABLE 2 Example No: 1 2 3 4 5 6 7 8 9 % by weight of 0 15 20 25 30 35 4045 100 polymer (B) % by weight of 100 85 80 75 70 65 60 55 0 polymer (A)MFI 190/5 0.49 0.31 0.3 0.295 0.285 0.27 0.26 0.24 0.16 [dg/min] MFI190/21.6 7.845 6.635 7.72 6.28 6.21 5.895 6.04 6.03 4.88 [dg/min] FRR*16.0 21.4 25.7 21.3 21.8 21.8 23.2 25.1 37.5 VN [cm³/g] 344 359 357 356377 395 390 393 486 *FRR = Ratio of MFI_(190/21.6) to MFI_(190/5)

The shear viscosities η of the mixtures of examples 1 to 9 weredetermined by the test method described above (ISO 6721, part 10), withangular frequency of 0.001 rad/s and angular frequency of 100 rad/s, andthe ratio η_(0.001 r/s)/η_(100 r/s) was then calculated. The results aregiven in table 3 below:

TABLE 3 η (0.001 rad/s) η (100 rad/s) η (0.001 rad/s)/ Example [Pa · s][Pa · s] η (100 rad/s) 1 2.25 · 10⁵ 2 450 91.8 2 2.28 · 10⁵ 2 500 91.2 32.32 · 10⁵ 2 556 90.7 4 2.78 · 10⁵ 2 530 109.8 5 2.76 · 10⁵ 2 570 107.46 3.55 · 10⁵ 2 540 139.8 7 4.02 · 10⁵ 2 550 157.6 8 4.86 · 10⁵ 2 550190.6 9 11.6 · 10⁵ 2 720 426.5

A glance at table 3 shows that the mixtures of examples 1 to 3 arecomparative examples in which the ratio of the shear viscositiesη_(0.001 r/s)/η_(100 r/s) determined at different angular frequencies isbelow 100. In contrast, examples 4 to 8 have results according to theinvention, and for these examples the ratio by weight of polymer (A) topolymer (B) is also in the range according to the invention, from 55 to75% by weight of polymer (A) and from 25 to 45% by weight of polymer B.

EXAMPLES 10 TO 12

To determine the homogeneity of the mixture (freedom from specks), thefollowing three further molding compounds were prepared:

Example 10 was the molding compound from example 1, i.e. pure polymer(A).

Example 11 was an in-situ reactor blend, i.e. a modified polymer (A), inwhich the amounts of ethylene in reactor 1 and reactor 2 were swappedduring the production process. 10 kg of ethylene/h were added withinreactor 1, and only 9 kg of ethylene/h within reactor 2, plus 260 ml/hof 1-butene as comonomer. The resultant modified polymer (A) had anMFI^(A′) _(5/190° C.) of 0.33 dg/min, and a density of 0.956 g/cm³, andcontained an amount of 1.7% by weight of comonomer, based on the totalweight of the higher-molecular-weight component.

Example 12 was a mixture made from 34% by weight of polymer (B) and 66%of polymer (A).

Polymer powder from examples 10 and 11 was pelletized in an extruder andthen processed to give blown films of thickness 5 μm The mixture ofexample 12 made from the powders of the polymers (A) and (B) was thenprepared in the same extruder at the same temperature and the sameoutput rate, and further processed by a similar method. The shearviscosities η of these molding compounds were then measured at thedifferent angular frequencies and their relationship determined, andhomogeneity (freedom from specks) was tested. The results from examples10 to 12 are given in table 4 below:

TABLE 4 Homogeneity to η (10⁻³ rad/s η (100 rad/s) η_(0.001 r/s)/ GKRguideline, Example [Pa · s] [Pa · s] η_(100 r/s) max. size*⁾ 10 1.70 ·10⁵ 2 570 66.1 0.013 11 2.55 · 10⁵ 2 400 106.3 0.014 12 3.75 · 10⁵ 1 980146 0.0010 *⁾Homogeneity is determined to the guideline of theGütegemeinschaft Kunststoffrohre [Quality association for plastic pipes]e.V. No. R 14.3.1 DA, 3.1.1.3.

Other properties of the polymers prepared in examples 10 to 12 are givenin table 5 below.

TABLE 5 Density MFR_(190/21.6) Viscosity number Example [g/cm³] [dg/min][ml/g] 10 0.954 9.2 330 11 0.956 9.52 370 12 0.954 8.8 340

It is entirely surprising to the skilled worker that a suddenimprovement in homogeneity and freedom from specks is given, at the sametemperature and the same throughput rate, only by the mixture of theinvention.

The test methods given in the description prior to the examples werethen also used to determine FNCT stress-cracking resistance [h] at atemperature of 95° C., and fracture toughness aFM [kJ/m²] at atemperature of 0° C. The results are given in table 6 below:

TABLE 6 aFM [kJ/m²] FNCT [h] Example 10 8.9 not determined Example 118.1 130.1 Example 12 10.6 175.0

Here again, it is clear that a step increase in FNCT stress-crackingresistance and, together with this, also a step increase in fracturetoughness aFM are given only by the mixture of the invention made fromethylene polymer A and ethylene polymer B in the mixing ratio foundaccording to the invention.

1. A polymeric molding compound made from a first ethylene polymer (A)and from a second ethylene polymer (B) which is particularly suitablefor producing thick-walled large-caliber pipes, wherein the moldingcompound comprises an amount in the range from 55 to 75% by weight ofthe first ethylene polymer (A) and an amount in the range from 25 to 45%by weight of the second ethylene polymer (B), based in each case on thetotal weight of the molding compound, where the first ethylene polymer(A) is a copolymer of ethylene with a 1-olefin having a total number ofcarbon atoms in the range from 4 to 10 as comonomer, and with aproportion of from 0.2 to 5% by weight of comonomer, based on the weightof the first ethylene polymer (A), with a wide bimodal molar massdistribution, and where the second ethylene polymer (B) is a copolymermade from ethylene units and from a 1-olefin having a number of carbonatoms in the range from 4 to 10, which has a bimodal molar massdistribution differing from that of the first ethylene polymer (A). 2.The polymeric molding compound as claimed in claim 1, which is preparedby mixing the mixing components, prepared separately from one another,the first ethylene polymer (A) and the second ethylene polymer (B), inan extruder in the form of an extruder blend.
 3. The polymeric moldingcompound as claimed in claim 1, which comprises a first ethylene polymer(A) with a density (measured at a temperature of 23° C.) in the rangefrom 0.94 to 0.96 g/cm³ and comprises a broad bimodal molar massdistribution, where the ratio, within the ethylene polymer (A), betweenthe weight of the low-molecular-weight fraction and the weight of thehigher-molecular-weight fraction is in the range from 0.5 to 2.0.
 4. Thepolymeric molding compound as claimed in claim 2, which comprises afirst ethylene polymer (A) with a density (measured at a temperature of23° C.) in the range from 0.94 to 0.96 g/cm³ and comprises a broadbimodal molar mass distribution, where the ratio, within the ethylenepolymer (A), between the weight of the low-molecular-weight fraction andthe weight of the higher-molecular-weight fraction is in the range from0.8 to 1.8.
 5. The polymeric molding compound as claimed in claim 1,wherein the first ethylene polymer (A) contains an amount from 0.2 to4.5% by weight of other comonomer units selected from the groupconsisting of 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, andmixtures of these.
 6. The polymeric molding compound as claimed in claim4, wherein the first ethylene polymer (A) contains an amount from 0.2 to4.5% by weight of other comonomer units selected from the groupconsisting of 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, andmixtures of these.
 7. The polymeric molding compound as claimed in claim1, which comprises, based on the total weight of the second ethylenepolymer (B), which has been prepared in the form of a reactor blend inthe presence of a Ziegler catalyst, and which comprises an amount in therange from 15 to 40% by weight of ultrahigh-molecular-weight ethylenehomopolymer as component (B¹) and comprises an amount in the range from60 to 85% by weight of low-molecular-weight copolymer with 1-butene ascomonomer in an amount of from 1 to 15% by weight, as component (B²). 8.The polymeric molding compound as claimed in claim 6, which comprises,based on the total weight of the second ethylene polymer (B), which hasbeen prepared in the form of a reactor blend in the presence of aZiegler catalyst, and which comprises an amount in the range from 15 to40% by weight of ultrahigh-molecular-weight ethylene homopolymer ascomponent (B¹) and comprises an amount in the range from 60 to 85% byweight of low-molecular-weight copolymer with 1-butene as comonomer inan amount of from 1 to 15% by weight, as component (B²).
 9. Thepolymeric molding compound as claimed in claim 7, wherein theultrahigh-molecular-weight ethylene homopolymer of component (B¹) has aviscosity number, VN^(B1), in the range from 1000 to 2000 cm³/g, andwherein the low-molecular-weight homopolymer of component (B²) has aviscosity number, VN^(B2), in the range from 80 to 150 cm³/g.
 10. Thepolymeric molding compound as claimed in claim 1, wherein the moldingcompound has fracture toughness aFM greater than or equal to 10 kJ/m².11. The polymeric molding compound as claimed in claim 1, wherein themolding compound has an FNCT stress-cracking resistance of ≧150 h. 12.The polymeric molding compound as claimed in claim 1, wherein themolding compound has shear viscosity, measured at 0.001 rad/s, is≧2.0·10⁵ Pa·s.
 13. The polymeric molding compound as claimed in claim 8,wherein the molding compound has shear viscosity, measured at 0.001rad/s, is ≧2.7·10⁵ Pa·s.
 14. The polymeric molding compound as claimedin claim 1, wherein the molding compound the viscosity ratio of theshear viscosities of the molding compound η_((0.001))/η₍₁₀₀₎ is greaterthan or equal to
 100. 15. A high-strength pipe made from the moldingcompound as claimed in claim 1, wherein the ethylene polymer A containscomonomers having from 4 to 6 carbon atoms, the amount being from 0 to0.1% by weight in the low-molecular-weight fraction and from 2.5 to 4%by weight in the higher-molecular-weight fraction, and has a melt flowindex MFI₅/_(190° C.) of ≦0.35 g/10 min.
 16. The pipe as claimed inclaim 15, wherein the pipe has a resistance to rapid crack propagation,measured to ISO/DIS 13477 on a pipe in pressure class PN 10 withdiameter 110 mm (S4 test) is greater than or equal to 20 bar.
 17. Amethod of transporting gases or water which comprises transporting thegases or the water through the pipe as claimed in claim 15.